1. Field of the Invention
This invention relates in general to the field of microelectronics, and more particularly to an apparatus and method for performing a cell selective soft handoff in a mobile telecommunications system that utilizes code division multiple access as the underlying communications method.
2. Description of the Related Art
The cell phone industry is undergoing exponential growth, not only in the this country, but all over the world. In fact, it is well known that the over twenty percent of the adult population in the United States do not even have a traditional landline telephone. In addition to those who do not own a conventional telephone, nearly ninety percent of the adult population owns a wireless phone.
And the usage of cell phones is increasing as well over the use of traditional landline telephone coverage. In fact, one in seven adults now uses only cell phones. Whereas in the past cell phones were used when a landline was not available or under emergency conditions, lower carrier rates, affordability of family packages, and free mobile-to-mobile or friend-to-friend promotions have fostered in significant increases in usage. It is not uncommon today to walk into any public forum or facility and notice a majority of the people there talking on their cell phones.
The ability to communicate using a mobile phone, or mobile station, has been available since the middle of the last century. However, during the 1990's so-called “2G” or second generation mobile phone systems were provided that began the growth in both deployment and usage that we currently enjoy today. These initial systems prevalently employed frequency division multiple access (FDMA) as the modulation strategy. One well known example of this technology is Global System for Mobile Communications (GSM), in which a particular cell phone communicates with its base station within a given cell over a specific frequency channel. In order for that cell phone to communicate with a base station in an adjacent cell, it must be assigned a new frequency channel and the switch coordinated in time with the old base station, the new base station, and the cell phone itself. This type of handoff from one cell to the next is known as a hard handoff.
Currently, so-called “3G” or third generation cellular communications technologies are being developed. These technologies are not characterized so much by modulation techniques, but more so by performance metrics (e.g., 2 Megabits per second indoor data rates, 384 Kilobits per second outdoor data rates). As such, there are presently a number of diverse approaches being proposed for 3G.
In the interim, a number of hybrid technologies are being provided, known as “2.5G” techniques and protocols, one of which is CDMA2000 1x. This system utilizes spread spectrum code division multiple access (CDMA) techniques to multiplex many move users over a single frequency channel that can be attained through the use of current FDMA systems. Under CDMA, the same frequency channel is used to communicate with all of the mobile stations within a group of adjacent cells and signals to and from the mobile stations are uniquely encoded using orthogonal codes.
Since the same frequency channel is used by adjacent base stations, this allows multiple base stations to provide for communications with a single cell phone, and as such, the phone is able to gracefully transition from one cell to the next. That is, two or more base stations in adjacent cells may be communicating with a single cell phone where their coverage areas overlap, but only one base station is designated as primary controller for the mobile station. As handoff to the next cell is coordinated between the base stations and the mobile station, it is realized often times in a coordinated change of designation within the cell phone itself. That is, rather than a “hard” switch from one frequency channel to the next, a “soft” switch is affected when the cell phone designates a base station with which it is currently communicating as its new primary base station.
The above advantages to the use of CDMA for cell handoff notwithstanding, the present inventors have observed that there are several problems and limitations to current approaches to cell handoff. One problem results from the fact that all base stations within a given area utilize the same frequencies to communicate. While one skilled in the art will appreciate that in the ideal a better estimation of signal content can be made when there are more signals received that carry the same information, a practical cell phone does not provide the capacity to process more than a fixed number of signals and thus, any signals that are weak (due to distance between a far base station and a given mobile) become difficult to process. Another problem is due to multipath effects. That is, reflected (i.e., delayed) copies of the same signal tend to add processing load on a mobile station, particularly when it is at the edges of adjacent cells. As has been observed, these delayed copies and weak transmissions from far base stations, rather than contributing to the overall reception quality of a mobile station within the network, have become problematic sources of interference, particularly when the mobile station transitions from one cell to the next.
Therefore, what is needed is an apparatus and method whereby weak signals from either far base stations or multipath sources can be identified and precluded in a CDMA handoff scenario.
In addition, what is needed is a mechanism in both mobile stations and base stations that allows for more efficient utilization of resources during a soft handoff.